Replies

1. 
   creativedestruction | Apr 05, 2021 10:44pm | #1

   The product link says 1 1/8" R5. You have zero outward drying potential with a foil faced product like that, so you have to have enough foam thickness for adequate condensation resistance at the sheathing. In zone 5B, 30% of overall R value is a good starting point for the foam layer(s):

   https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies

   2x8 dense packed is somewhere around R26-27, so R11+ in foam is necessary -- OR -- stud depth could be reduced to require less layers of foam. All depends on your overall target R value.

   If cost is the primary concern, EPS would be a better option. Sure, the foil faced GPS will give you a better taping surface if you want that, but when it's installed over zip sheathing I'm not sure I'd go to the trouble.

1. 
   creativedestruction | Apr 07, 2021 05:20pm | #3

   Amending my first comment. Confused it with a similar product. Not foil faced, and the manufacturer claims permeability via perforations, so it has some drying potential. Still a good idea to obey the 30% ratio if possible.

2. 
   patrick1 | Apr 07, 2021 04:40pm | #2

   I had been looking at that product as well for some attic insulation work.

   The graphite is supposed to make the EPS have a higher insulation value per inch. Depending on cost though, you might also want to consider the higher density SilverBoard products without the graphite. These can get you R5 per inch instead of R4.5 and unlike XPS and Polyiso, there's no issues with long-term degradation or thermal drift of the R-value:

   https://www.amvicsystem.com/products/hd-foam/silverboard/

   In my area, the prices are pretty similar between the two products.